The invention relates to a mobile data communication system of a type comprising a fixed station and at least two mobile stations, the fixed station and the mobile stations each comprising a transmitter and a receiver, the transmitter of the fixed station consecutively interrogating the mobile stations, the receiver of each mobile station being synchronized with messages transmitted by the transmitter of the fixed station, and the receiver of the fixed station being synchronized with reply messages consecutively transmitted by the transmitters of the mobile stations, by applying to a control unit of a controllable divider of a clock signal generator a control signal produced by measuring in a control loop the bit phase difference between each reply message and the clock signals produced by the divider.
Systems of the above type are known wherein a fixed station consecutively interrogates mobile stations comprised by a fleet of vehicles, for example motorbuses. The fixed station transmits uninterruptedly messages which can be received by all vehicles. Each message comprises an unique code so that only the vehicle which responds to the relevant code processes the message and returns a reply message. In this manner the fixed station exchanges messages in a prescribed sequence with the vehicles of the fleet. The fixed station and the vehicles each comprise a transceiver. The receiving frequency of the vehicles is the same as the transmitting frequency of the fixed station and the receiving frequency of the fixed station is the same as the transmitting frequency of the vehicles. As the receivers of the vehicles receive the transmission from the fixed station continuously, bit synchronisation of the receivers of the vehicles with the transmitter of the fixed station is ensured. In contrast the receiver of the fixed station must be separately synchronized for accessing each vehicle in order to receive the reply message transmitted by the relevant vehicle. The reply messages are therefore so arranged that they include synchronisation signals. A drawback of this is that it requires additional transmission time for the reply messages from the vehicles. As a consequence, the interrogation frequency of the fixed station is limited. To satisfy the frequently imposed requirement that it must be possible to exchange a message with each vehicle once in a specified unit of time, it is therefore necessary to limit the number of vehicles which can be "served" by each fixed station.